Re: [libvirt] [dpdk-dev] dpdk/vpp and cross-version migration for vhost

On 11/24/2016 12:47 PM, Maxime Coquelin wrote: > > > On 11/24/2016 01:33 PM, Yuanhan Liu wrote: >> On Thu, Nov 24, 2016 at 09:30:49AM +0000, Kevin Traynor wrote: >>> > On 11/24/2016 06:31 AM, Yuanhan Liu wrote: >>>> > > On Tue, Nov 22, 2016 at 04:53:05PM +0200, Michael S. Tsirkin wrote: >>>>>>> > >>>> You keep assuming that you have the VM started first and >>>>>>> > >>>> figure out things afterwards, but this does not work. >>>>>>> > >>>> >>>>>>> > >>>> Think about a cluster of machines. You want to start a VM in >>>>>>> > >>>> a way that will ensure compatibility with all hosts >>>>>>> > >>>> in a cluster. >>>>>> > >>> >>>>>> > >>> I see. I was more considering about the case when the dst >>>>>> > >>> host (including the qemu and dpdk combo) is given, and >>>>>> > >>> then determine whether it will be a successfull migration >>>>>> > >>> or not. >>>>>> > >>> >>>>>> > >>> And you are asking that we need to know which host could >>>>>> > >>> be a good candidate before starting the migration. In such >>>>>> > >>> case, we indeed need some inputs from both the qemu and >>>>>> > >>> vhost-user backend. >>>>>> > >>> >>>>>> > >>> For DPDK, I think it could be simple, just as you said, it >>>>>> > >>> could be either a tiny script, or even a macro defined in >>>>>> > >>> the source code file (we extend it every time we add a >>>>>> > >>> new feature) to let the libvirt to read it. Or something >>>>>> > >>> else. >>>>> > >> >>>>> > >> There's the issue of APIs that tweak features as Maxime >>>>> > >> suggested. >>>> > > >>>> > > Yes, it's a good point. >>>> > > >>>>> > >> Maybe the only thing to do is to deprecate it, >>>> > > >>>> > > Looks like so. >>>> > > >>>>> > >> but I feel some way for application to pass info into >>>>> > >> guest might be benefitial. >>>> > > >>>> > > The two APIs are just for tweaking feature bits DPDK supports >>>> before >>>> > > any device got connected. It's another way to disable some features >>>> > > (the another obvious way is to through QEMU command lines). >>>> > > >>>> > > IMO, it's bit handy only in a case like: we have bunch of VMs. >>>> Instead >>>> > > of disabling something though qemu one by one, we could disable it >>>> > > once in DPDK. >>>> > > >>>> > > But I doubt the useful of it. It's only used in DPDK's vhost >>>> example >>>> > > after all. Nor is it used in vhost pmd, neither is it used in OVS. >>> > >>> > rte_vhost_feature_disable() is currently used in OVS, >>> lib/netdev-dpdk.c >> Hmmm. I must have checked very old code ... >>> > >>> > netdev_dpdk_vhost_class_init(void) >>> > { >>> > static struct ovsthread_once once = OVSTHREAD_ONCE_INITIALIZER; >>> > >>> > /* This function can be called for different classes. The >>> > initialization >>> > * needs to be done only once */ >>> > if (ovsthread_once_start(&once)) { >>> > rte_vhost_driver_callback_register(&virtio_net_device_ops); >>> > rte_vhost_feature_disable(1ULL << VIRTIO_NET_F_HOST_TSO4 >>> > | 1ULL << VIRTIO_NET_F_HOST_TSO6 >>> > | 1ULL << VIRTIO_NET_F_CSUM); >> I saw the commit introduced such change, but it tells no reason why >> it was added. > > I'm also interested to know the reason. I can't remember off hand, added Mark K or Michal W who should be able to shed some light on it.

> In any case, I think this is something that can/should be managed by > the management tool, which should disable it in cmd parameters. > > Kevin, do you agree? I think best to find out the reason first. Because if no reason to disable in the code, then no need to debate! > > Cheers, > Maxime